Rainwater Collection Calculator
Calculate how much rainwater you can harvest from your roof
Inputs
Footprint of roof draining to collection
Average yearly rainfall in your area
Metal roofs ~90%, shingles ~75%
Collection per inch of rain
Roof efficiency
US rainfall averages
How to use this calculator
- 1Measure or estimate your roof collection area (footprint, not surface)
- 2Look up average annual rainfall for your location
- 3Estimate collection efficiency based on your roof type
- 4View your potential annual rainwater harvest
Rainwater harvesting tips
A 1,000 sq ft roof can collect about 600 gallons from just 1 inch of rain. Metal roofs have the highest collection efficiency (90%+).
First flush diverters help improve water quality by discarding the first few gallons that wash debris off the roof.
Storage sizing: Size your tank based on the longest dry spell in your area. In many regions, 2-4 weeks of storage covers dry periods between rain events.
Frequently asked questions
How do I calculate roof collection area?
Use the footprint (horizontal area), not the actual roof surface area. A 30x40 foot house has 1,200 sq ft of collection area regardless of roof pitch. Include all areas that drain to collection.
What affects collection efficiency?
Metal roofs are most efficient (90%+). Asphalt shingles lose more to absorption (75-80%). First flush diverters, gutter losses, and evaporation also reduce total collection.
Is rainwater safe to drink?
Rainwater should be filtered and treated before drinking. A basic system includes first flush diverter, sediment filter, and UV treatment or chlorination. Always test water quality.
Result
19,936gallons
Total yearly potential